Large concrete structures using drilled shaft foundations are often cast in place. In some cases these foundation elements have been constructed without considering mass concrete effects and the possible long-term implications of the concrete integrity. Such considerations address the extremely high internal temperatures that can be generated during the concrete hydration/curing phase. The extremely high internal temperatures can be detrimental to the shaft durability and/or integrity in two ways: (1) short-term differential temperature-induced stresses that crack the concrete and (2) long-term degradation via prolonged excessively high temperatures while curing.
Mass concrete is generally considered to be any concrete element that develops differential temperatures between the innermost core and the outer surface, which can develop tension cracks due to the differential temperatures. Some state departments of transportation (DOTs) have defined geometric guidelines that identify potential mass concrete conditions as well as limits on the differential temperature experienced. For instance, the Florida DOT designated any concrete element with minimum dimension exceeding 0.91 m (3 ft) and a volume to surface area ratio greater than 0.3 m3/m2 will require precautionary measures to control temperature-induced cracking (FDOT, 2006). The same specifications set the maximum differential temperature to be 20° C. (35° F.) to control the potential for cracking. For drilled shafts, however, any element with diameter greater than 1.83 m (6 ft) is considered a mass concrete element despite the relatively high volume to area ratio.
The latter of the two integrity issues, i.e., excess high temperature, is presently under investigation at a number of institutions. When concrete temperature exceeds safe limits on the order of 65° C. (150° F.), the concrete may not cure correctly and can ultimately degrade via latent expansive reactions termed delayed ettringite formation (DEF). This reaction may lay dormant for several years before occurring; or the expansion may not occur as it depends on numerous variables involving the concrete constituent properties and environment.
Accordingly, there is a need for providing cast-in-place foundation structures that can reduce or eliminate durability and integrity issues associated with excess high temperatures.